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303                  PA THOGENIC BA CTERIA.

The Preparation of the Serum for Therapeittic Pur-
poses.—When, because of the tolerance to large quanti-
ties of toxin, the horse seems to possess antitoxic blood,
a l' twitch'J is applied to the upper lip, the eyes are
blindfolded, a small incision is made through the skin,
a trocar thrust into the jugular vein, and the blood al-
lowed to flow through a cannulated tube into sterile
bottles. It is allowed to coagulate, and kept upon ice
for two days or so, that the clear serum may be pi-
petted off. This serum is the antitoxic seriim. It does
not always materialize according to the ^desires of the
experimenter, sometimes proving surprisingly strong in
a short time, sometimes very weak after months of
patient preparation.

The serums are preserved by Roux with camphor, by
Behring with carbolic acid (0.5 per cent), and by Aron-
son with trikresol (0.4 per cent.). I prefer to use tri-
kresol, is not poisonous, is a reliable antiseptic, and
has a very pronounced local anesthetic action. Formalin
has been tried, but it gelatinizes the serum and causes
much local pain when injected beneath the skin.

Dried antitoxic serum has also been placed upon the
market under the impression that it will keep longer and
bear shipment better than any other. This is not, how-
ever, shown to be the case, and as the dried serum dis-
solves with difficulty it is much less convenient than the
usual preparations. It is also less likely to be sterile
than the liquid forms.

The strength of the serum is expressed in what are
known as immunising units. This denomination origin-
ated with Behring and Ehrlich, whose normal serum
was of such strength that o.i of it would protect
against ten times the least certainly fatal dose of toxin
when simultaneously injected into guinea-pigs. Each
cubic centimeter of this normal serum they called an
'immtinizing unit. Later it was shown that the strength
of the serum could easily be increased tenfold, so that
o.oi of the serum would protect the guinea-pig